The present invention relates to a semiconductor device having an opening portion for breaking a fuse and, particularly, to a structure of the opening portion. Description of the related Art
It has been known that some of semiconductor devices such as DRAMs, SRAMs or ROMs, etc., are constructed such that, in order to improve the yield of products, extra memory cells, that is, redundancy memory cells in addition to a predetermined number of memory cells are provided and, when a specific memory cell of the memory cells, whose performance is not acceptable, is found in a fabrication thereof, the specific memory cell can be replaced by one of the redundancy cells.
In such semiconductor device, in order to replace such defective memory cell by the redundancy memory cell, redundancy fuses for replacement of the usual memory cell by the redundancy memory cell are buried therein so that the defective memory cell is disconnected by cutting away a redundancy fuse corresponding to the defective memory cell by laser light, etc., and the redundancy memory cell is connected in place.
On the other hand, in the semiconductor device having the redundancy fuses buried therein, a multi-layer wiring has been employed in order to accommodate it to the increased integration and density. When the multi-layer wiring is employed, an overcoat covering the multi-layer wiring should be as flat as possible in view of prevention of breakage of wiring. Therefore, the overcoat is usually formed by an SOG (Spin On Glass) film, that is, a silica film, which is superior in flatness and covering ability. As such, in a semiconductor device which uses an SOG film as the overcoat film between wiring, an opening portion is formed on an insulating film of BPSG, etc., formed on a redundancy fuse buried therein, which opening portion has a depth not enough to expose the redundancy fuse, to facilitate the breakage of the redundancy fuse by laser light. In such case, a portion of the SOG film is exposed on a side face of the opening portion on the redundancy fuse.
However, the SOG film itself has a tendency of absorbing water with which it expands. When the SOG film on the side face of the opening is kept exposed, it absorbs water and expands during a use thereof, resulting in cracks in the wiring, etc. Further, there may be a case of wiring corrosion due to existence of water.
In order to prevent the SOG film from absorbing water, a semiconductor device has been proposed, in which the exposure of an SOG film is prevented by providing an insulating side wall of SiN on the exposed side face of the opening portion on an area in which the redundancy fuse is buried.
In the latter case, the formation of the SiN insulating side wall on the exposed portion of the SOG film is performed by forming an SiN film on a whole surface of a wafer including the opening portion and etching it back to remove a portion of the SiN film left on the wafer except the side face portion of the opening portion.
Therefore, in this example, in addition to the step of patterning a cover resist painted to form the opening, the step of depositing an insulating film of such as SiN which has no relation to the wiring and the step of etching back the insulating film are required in order to form the side wall on the SOG film. The latter two steps for forming the insulating film for covering the exposed portion of the SOG film are performed separately from the steps for forming the wiring. Therefore, there is a defect in that the fabrication of the semiconductor device becomes complicated.
Further, in order to prevent water permeation from the exposed SOG film in the opening portion into internal wiring, a semiconductor device having a dummy wiring surrounding the opening portion has been proposed. In such construction, it is possible to form the dummy wiring and other wiring on a single and same layer, so that the fabrication of the semiconductor device can be simplified. However, in order to prevent the water permeation from the exposed SOG film in the opening portion, it is necessary to make a width of the dummy wiring larger to provide a sufficient margin. Therefore, an increase of area due to the dummy wiring having large width is indispensable. Even so, however, it has been observed that water still permeates from the exposed portion of the SOG film along the SOG film into the wiring.